Stem cells have the ability to self-renew and to differentiate into functionally different cell types. They have the potential to be a powerful therapeutic tool, for example in the growing field of Regenerative Medicine, in particular regenerative therapy requiring tissue replacement, regeneration or repair (Banerjee et al. 2011). Endogenous stem cells have also been implicated as targets (endogenous “cancer stem cells”) of anti-cancer therapy, where it is proposed to treat the cancer by eliminating the cancer stem cells that are thought to drive cancer growth and metastasis. More recently, engineered mesenchymal stem cells have been proposed as delivery vehicles in anti-cancer therapy (Dai et al., 2011; Shah et al. 2012). However, there are drawbacks to the use of stem cells in therapy: there is a need for a consistent and substantial supply of stem cells with functional and phenotypic stability and the associated high costs and time delay caused by cell generation, storage, transport and handling; there is a requirement for immunological compatibility to avoid rejection of the stem cells by the recipient; and there are complex regulatory issues related to potential safety risks of tumour or ectopic tissue formation. Further, despite the therapeutic efficacy of stem cell transplantation, there is no convincing evidence for a direct long-term effect of the transplanted stem cells, for example through engraftment and differentiation into reparative or replacement cells.
Neural stem cells (NSCs) are self-renewing, multipotent stem cells that generate neurons, astrocytes and oligodendrocytes (Kornblum, 2007). The medical potential of neural stem cells is well-documented. Damaged central nervous system (CNS) tissue has very limited regenerative capacity so that loss of neurological function is often chronic and progressive. Neural stem cells (NSCs) have shown promising results in stem cell-based therapy of neurological injury or disease (Einstein et al. 2008). Implanting neural stem cells (NSCs) into the brains of post-stroke animals has been shown to be followed by significant recovery in motor and cognitive tests (Stroemer et al. 2009). It is not completely understood how NSCs are able to restore function in damaged tissues but it is now becoming increasingly recognised that NSCs have multimodal repairing properties, including site-appropriate cell differentiation, pro-angiogenic and neurotrophic activity and immunomodulation promoting tissue repair by the native immune system and other host cells (Miljan & Sinden, 2009, Horie et al., 2011). It is likely that many of these effects are dependent on transient signalling from implanted neural stem cells to the host milieu, for example NSCs transiently express proinflammatory markers when implanted in ischaemic muscle tissue damage which directs and amplifies the natural pro-angiogenic and regulatory immune response to promote healing and repair (Katare et al., Clinical-grade human neural stem cells promote reparative neovascularization in mouse models of hindlimb ischemia. Arteriosclerosis, Thrombosis and Vascular Biology, vol 34, no. 2, pp. 408-418). In chronic stroke brain, NSCs also have a substantial neurotrophic effect. For example, they promote the repopulation of the stoke-damaged striatal brain tissue with host brain derived doublecortin positive neuroblasts (Hassani, O'Reilly, Pearse, Stroemer et al., PLoS One. 2012; 7(11)).
Furthermore, on the basis of a large body of NSC restorative effects in animal models with chronic stroke, a clinical trial using neural stem cells is being carried out by ReNeuron Limited (Surrey, UK), to trial the treatment of disabled stroke patients using its “CTX0E03” conditionally-immortalised cortex-derived neural stem cells (Clinicaltrials.gov Identifier: NCT01151124).
Mesenchymal stem cells (MSCs) are lineage-restricted stem cells which have the potential to differentiate into mesenchymal cell types only, namely of the adipocytic, chondrocytic and osteocytic lineages (Pittenger et al. 1999; Ding et al. 2011). MSCs (also referred to as Mesenchymal Stromal Cells and Mesenchymal Progenitor Cells) are derived from a variety of sources including bone marrow, blood, adipose and other somatic tissues. The therapeutic potential of MSCs, however, is more directed towards the application of their pro-angiogenic and immune modulating properties as undifferentiated cells. Production of human MSCs is limited by the inability of these cells to expand in numbers stably beyond approximately 15-20 population doublings.
Mesenchymal stem cell-conditioned medium (MSC-CM) has a therapeutic efficacy similar to that of MSCs themselves, suggesting a paracrine mechanism of MSC-based therapy (Timmers et al. 2007). WO-A-2009/105044 discloses that particles known as exosomes, secreted by MSCs, comprise at least one biological property of the MSCs and suggests the use of these MSC particles in therapy, while Théry et al. 2011 provides a general review of exosomes and other similar secreted vesicles. Whereas some of the drawbacks of using stem cells directly as therapeutic agents are overcome by using the mesenchymal stem cell-derived exosomes (e.g. storage, transport and handling), the problem remains of providing a consistent and substantial supply of functionally and phenotypically stable stem cells to produce the exosomes. For therapeutic use, the exosomes preferably need to be produced on a large scale. In the absence of a stem cell line, replenishment of the cells through repeated derivation from a source of stem cells is required, which incurs recurring costs for testing and validation of each new batch. Furthermore, the diseases and disorders that can be treated by MSCs may be limited.
WO-A-2013/150303 and WO-A-2014/013258 disclose microparticles produced by neural stem cells, methods for making those microparticles and uses of those microparticles, in particular for use in regenerative therapy.
There remains a need for improved stem cell-based therapies.